Aug. 19O 5, 29. on the GEOLOGY of ARENIG FAWR A~*D MOEL L~.Y:F
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Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016 608 ~ta. w. ~. F~An~SrD~S 0.~ Tr~E Gv.O~0OY oF [Aug. 19o 5, 29. On the GEOLOGY of ARENIG FAWR a~*d MOEL L~.Y:F~A:Sr. By WILLIAm1 GEOR6E FEARNSlDES,M.A., F.G.S., Fellow of Sidney Sussex College, Cambridge. (Read January 18th, 1905.) [P~w. XLI--MAP.] l. INTRODUCTION. T~ mountains known as Arenig Fawr and Moel Llyfnant are situated somewhat to the north of the centre of the county of Merioneth, and lie immediately east of the main water-parting of Wales. The area studied lies wholly within the quarter-sheets 13 S.E., 13 S.W., 21 IN.E, & 21 I~.W. of the 6-inch Ordnance- Survey maps of Merionethshire. Rising from a plateau some 800 to 1200 feet high, they attain heights of 2800 and 2437 feet respectively, and, being entirely above the cultivation-zone, are practically uninhabited. The Great Western ]3ala-Ffestiniog Railway crosses the northern end of the district, to which Arenig Station, close under the northern slopes of Arenig Fawr, affords easy access. Although to a certain extent glaciated, the mountains are swept clean rather than Drift-covered, and it is only in the lower ground that moraines become inconveniently abundant. The adaptation of surface-forms to rock-structures is exceedingly complete, and few are the places where the ribs of harder rocks do not protrude through the scanty soil or turf. Being open to the full blast of the westerly and south-westerly gales from the ocean, the district, receives its full share of weather ; and, in consequence, ahhough the eastern slopes are often grass- or heather-covered, the steeper parts of the western slopes are rugged and bare, and afford magnificent exposures of the component rocks. Further, the structure of the district is fairly simple, and the cleavage, though present, is not s(> strongly developed as to prevent one from breaking the rock along the bedding-planes. :Ever since the time when Sedgwick applied the name of Arenig Ashes and Porphyries to the lower series of h'orth Welsh volcanic rocks, the Arenig district has been a source of interest to geologists. Yet, and notwithstanding that the name of Arenig is so constantly upon the lips of all who study or teach the geology of the Lower Palteozoic rocks, no complete or detailed account of it has yet been published. The only connected general account of the district which I have found, is that contained in the first edition of Sir Andrew t~amsay's ' Geology of :North Wales. '~ A good deal of additional information is contained in the second edition of that work (.1881), but is somehow 1 Mere. Geol. Surv. ~ol. iii (1866) Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016 Vo1. 6 I.] ARENIG FAWR AND MOEL LLYFNANT. 609 less easy to extract. Much more instructive are the few pages devoted by ~essrs. A. V. Jennings & G. J. Williams ~ to the Garth Grit, and the recent paper by Miss G. L. Elles ~ on the general pal~eoutology of the Arenig rocks of Wales. Passing now from printed matter, I take this opportunity of acknowledging my indebtedness to Prof. T. McKenny Hughes. F.R.S., and also to Mr. G. J. Williams, F.G.S., H.M. Assistant- Inspector of Metalliferous Mines, of Bangor. Prof. Hughes has, in the course of the last twenty years, made frequent excursions to Arenig, alone and also as leader of large classes of Cambridge students. He has in this way collected a vast amount of ex- tremely useflfl material, and all this, together with the corresponding field-notes and sketch-real)S, he most generously placed at my disposal. Mr. G. J. Williams was long resident at Ffestiniog, which is only about a dozen miles away, and more than twenty years ago discovered the fascinations of the Arenig district as a collecting- ground. Unfortunately, change of residence and increased pressure of work have latterly prevented him from continuing his researches, which have never been concluded; and when I announced my intention of mapping the whole comJtry on the 6-inch scale, he kindly gave me a day over the ground, pointing out fossil-localities. Moreover, since that time he has always allowed me full access to his fine and carefully-localized collection of Welsh fossils. II. DESCRII'~ION OF THE ROcK-SuccEssioN. The general succession (see also figs. I & 2, pp. 610-11) of the sedimentary rocks of the district is as follows :-- Dieranograptas-Shales. Ga r a d o O. Derfel or OrtMs-Limestone. J Rhyolitic Ashes = Upper ~ Upper Ashes of "~ Massive Ashes = Middle f Arenig. ] Aeid Andesitic Ashes = Lower Daerfawr Shales. Zone of Di@mograptus Murchisoni. ~ Llandeilo. Platy Ashes. ) Lower (Hypersthene-Andesites) ! Great Agglomerate. ~ Ashes or' Arenig. ) Olehfi~ or B~dus-Shales. Zone of Didumograpt~s 9 bifidus. [ Filltirgerig or ttiru~do-Beds. ~ Zone of JDidymograpgus Erwent or Ogygia.Limestone. ~ hirundo. ~- Arenig. ]~enllan or Calymene-Ashes. BasalLlyfnantGrit.~Exte~tsu~-Fiags. t Zoneextensus.OfDid~mograptus )t Uncontbrmity. l, Manod & the Moelwyns' Quart. Journ. Geol. Soc. vol. xlvii (189l) pp. 373-74. 2 Geol. Mag. 1904, lop. 199-211. Q. J. G.S. :No. 243. 2 u Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016 r--.o ~J ~q ~o Lmz % ~ ~176~ u~ Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016 e~g 0 ~.," ~ =L . ~'~ ~ ~'~'~'r~ II l[ [] [I li I[ [] I[ [I .= ..~ ~ ~.~ .~. m ~ 0 ~ ca 0 "~c-.a~ .-.9. -. e 11 [I I] [] l! [I I[ II ~ t r ~ << .. ~,~'~--= ~ li II II II II II I1 II ~ ~1 .,~ -'1" ~ e.~ Its- O0 "2 t7 2 Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016 612 ~IR. W. O. FEARNSIDES ON THE GEOLOGY OF [Aug. I9O5, Amnodd or Shumardia-Sh'ales. Tai-tlerion or Asaphellus-Flags, I Tremadoc. Nant-ddu or Belle~,~2ho~t-Beds. Dic@o~ema-Band. 1 l~obe- and PsiloceThal,us-Beds. { PelCura-Beds. 1 Orlhis-le~tieularis Band. f Black Band. Dolgelly. Paraboli~m-Bcds. Li~lyu{e[[a.Beds. [Ffestiniog. Grits and Flags. J There are Mso important intrusions of andesites (ahnost identical with tho Lower Ashes of Arenig), hornblende-porphyrites, and ophitic dolerites. The Ffestiniog Beds [22]. The oldest rocks of the district~ occur in the west and south-west, and belong to the shallow-water phase of the Linfpdct-Flags. The) are a monotonous series of grits and flags with hardly any fossils, and determine a belt of most uninteresting, boggy, heather-covered, rolling moorland of only slight relief. Westward they. rise to form the steep escarpment which locally determines the main watershed of Wales. Although usually dipping" eastward or north-eastward at about~ 30 ~ they are so broken up by folds and faults of small amplitude, that, in the absence of any continuous exposure, it is impossible to form any exact estimate of their thickness; but, as the outcrop is considerably more than a mile wide, that, thickness must be considerable. The beds are best. exposed in the valley of the Lliw, where also may be seen a fine large sill of hornblende-porphyrite, quite like the well-known sills which occur at about the same horizon at Dolgelly. The Linqulella-Beds [21 ], Towards the top these grits and flags become much finer in texture, and in the highest 30 or 40 feet are crowded with specimens of Li~gulella Davisii, M'Coy, which, in places, beconie so abundant as to form the greater part. of certain bands (4(~37). l These beds are exceedingly easy to recognize by their mode of weathering, for, fi'om being bluish-grey, they become first quite rusty and brown and then increasingly paler, until finally the 5" are almost white. A similar belt of fossiliferous flags occurs in the same relative position at Penmorfa (Portmadoc) and at lthobell Fawr ; hence it would seem that the division should be traceable all round the Itarlech dome. The beds are well exposed in the two westernmost branches of the Trinant Valley. The Parabolina-Beds [20]. Passing upward, the Lb~gulella-Beds become finer-grained and less gritty, giving place gradually to the hard flaggy shales of lhe x The numerals in parentheses throughout tlle text of this paper refer to the numbers of tile correspol~ding r(~ek-specimens preserved in the Sedgwick Museum at Cambridge. Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016 Vol. 6I.i] *RE.~IG rXWR .~XD ~OEr, rLX'FXAXX. 613 Pcwabolina-Beds. These are dark bluish-grey, and give a sonorous ring when struck ; they break with a curious, china-like, conchoidal fracture, and tend to fly before the hammer. They contain a certain amount of iron-pyrites, and coat themselves with yellow rust in the early stages of weathering, although this afterwards disappears and ti~e whole rock appears much darker. In the lower parts they are quite unfossiliferous, but about the middle certain bedding-planes yield indifferently-preserved examples of Par(lbolina ,~pinulosa, Wahl., which becomes abundant in the higher beds. The Orthis-lenticularis Band [19~. Some 200 feet from the base of the Farabolimt-Beds appears a band of darker and more earthy shale crowded with innumerable shells of Orthis lentic,bwis, 1)ahn. This band is only some 4 or 5 feet thick, but is readily recognized at intervals all round the Harlech dome from Ogof-ddu near Criccieth to Dolgelly.